When the multiverse and many-worlds collide

TWO of the strangest ideas in modern physics – that the cosmos constantly splits into parallel universes in which every conceivable outcome of every event happens, and the notion that our universe is part of a larger multiverse – have been unified into a single theory. This solves a bizarre but fundamental problem in cosmology and has set physics circles buzzing with excitement, as well as some bewilderment.

The problem is the observability of our universe. While most of us simply take it for granted that we should be able to observe our universe, it is a different story for cosmologists. When they apply quantum mechanics – which successfully describes the behaviour of very small objects like atoms – to the entire cosmos, the equations imply that it must exist in many different states simultaneously, a phenomenon called a superposition. Yet that is clearly not what we observe.

Cosmologists reconcile this seeming contradiction by assuming that the superposition eventually “collapses” to a single state. But they tend to ignore the problem of how or why such a collapse might occur, says cosmologistRaphael Bousso at the University of California, Berkeley. “We’ve no right to assume that it collapses. We’ve been lying to ourselves about this,” he says.

In an attempt to find a more satisfying way to explain the universe’s observability, Bousso, together with Leonard Susskind at Stanford University in California, turned to the work of physicists who have puzzled over the same problem but on a much smaller scale: why tiny objects such as electrons and photons exist in a superposition of states but larger objects like footballs and planets apparently do not.

This problem is captured in the famous thought experiment of Schrödinger’s cat. This unhappy feline is inside a sealed box containing a vial of poison that will break open when a radioactive atom decays. Being a quantum object, the atom exists in a superposition of states – so it has both decayed and not decayed at the same time. This implies that the vial must be in a superposition of states too – both broken and unbroken. And if that’s the case, then the cat must be both dead and alive as well.

To explain why we never seem to see cats that are both dead and alive, and yet can detect atoms in a superposition of states, physicists have in recent years replaced the idea of superpositions collapsing with the idea that quantum objects inevitably interact with their environment, allowing information about possible superpositions to leak away and become inaccessible to the observer. All that is left is the information about a single state.

Physicists call this process “decoherence”. If you can prevent it – by tracking all the information about all possible states – you can preserve the superposition.

In the case of something as large as a cat, that may be possible in Schrödinger’s theoretical sealed box. But in the real world, it is very difficult to achieve. So everyday cats decohere rapidly, leaving behind the single state that we observe. By contrast, small things like photons and electrons are more easily isolated from their environment, so they can be preserved in a superposition for longer: that’s how we detect these strange states.

The puzzle is how decoherence might work on the scale of the entire universe: it too must exist in a superposition of states until some of the information it contains leaks out, leaving the single state that we see, but in conventional formulations of the universe, there is nothing else for it to leak into.

What Bousso and Susskind have done is to come up with an explanation for how the universe as a whole might decohere. Their trick is to think of the volume of space that encompasses all the information in our universe and everything it might possibly interact with in the future. In previous work, Susskind has dubbed this region a causal patch. The new idea is that our universe is just one causal patch among many others in a much bigger multiverse.

Many physicists have toyed with the idea that the cosmos is made up of regions which differ so profoundly that they can be thought of as different universes inside a bigger multiverse. Bousso and Susskind suggest that information can leak from our causal patch into others, allowing our part of the universe to decohere into one state or another, resulting in the universe that we observe.

But while decoherence explains why we don’t see cats that are dead and alive at the same time, or our own universe in a huge superposition of states, it does not tell us which state the cat, or the universe, should eventually end up in. So Bousso and Susskind have also linked the idea of a multiverse of causal patches to something known as the “many worlds” interpretation of quantum mechanics, which was developed in the 1950s and 60s but has only become popular in the last 10 years or so

Bousso and Susskind’s contention is that the alternative realities of the many worlds interpretation are the additional causal patches that make up the multiverse. Most of these patches would have split from other universes, perhaps even ancestors of our own. “We argue that the global multiverse is a representation of the many-worlds in a single geometry,” they say. They call this idea the multiverse interpretation of quantum mechanics and in a paper now available online they have proposed the mathematical framework behind it (arxiv.org/abs/1105.3796).

One feature of their framework is that it might explain puzzling aspects of our universe, such as the value of the cosmological constant and the apparent amount of dark energy.

The paper has caused flurry of excitement on physics blogs and in the broader physics community. “It’s a very interesting paper that puts forward a lot of new ideas,” says Don Page, a theoretical physicist at the University of Alberta in Edmonton, Canada. Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena and author of the Cosmic Variance blog, thinks the idea has some merit. “I’ve gone from a confused skeptic to a tentative believer,” he wrote on his blog. “I realized that these ideas fit very well with other ideas I’ve been thinking about myself!”

However, most agree that there are still questions to iron out. “It’s an important step in trying to understand the cosmological implications of quantum mechanics but I’m sceptical that it’s a final answer,” says Page.

For example, one remaining question is how information can leak from a causal patch, a supposedly self-contained volume of the multiverse.

Susskind says it will take time for people to properly consider their new approach. And even then, the ideas may have to be refined. “This is not the kind of paper where somebody does a calculation and confirms that we’re correct,” says Bousso. “It’s the sort of thing that will take a while to digest.”

One thought on “Why do we see our world as we do”

There’s a crucial point that they don’t tell you. It explains why physicists have been putting unobservable other universes into their theories since the early 1980s. Physicists don’t add bits to a theory which make it untestable, or more or less untestable, unless they have a very good reason. It then risks being a hypothesis rather than a theory – a good theory is very testable. But since 1979 when the first paper on the fine tuning for life was published by Bernard Carr and Martin Rees, it has become clear that our universe – with its set of laws of physics that are so suitable for intelligent life – came about in one of two ways. Either that trillion to one event came about by the intention of some intelligence, with the aim that life should evolve later, or if not, there are many universes beyond our own, with varying laws of physics, to make up the odds. This also explains why, as the article says, Everitt’s many worlds theory from the 1950s has only become popular in the last ten years or so. Read “Universe or multiverse” edited by Bernard Carr, which is the proceedings from two conferences on the fine tuning, published in 2007. Hawking, Smolin, Barrow, Davies, Weinberg and about 20 others – all the world’s top thinkers – each has about ten pages on the subject, and the underlying implication is that there are now only these two basic possibilities.